Why does MQ SYSTEM.CLUSTER.TRANSMIT.QUEUE have messages after queue manager is removed from the cluster?

We followed the documented steps to remove our WebSphere MQ queue manager (QM1) from cluster (CLUSTER1) and then removed the cluster. A few weeks later, we noticed there are some messages in the SYSTEM.CLUSTER.TRANSMIT.QUEUE on QM1 that are intended to go to CLUSTER1, the one that we removed. What are these messages and what do we do with them?

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The messages in QM1's SYSTEM.CLUSTER.TRANSMIT.QUEUE which have references to CLUSTER1 are subscription messages and will not affect the remaining clusters. They can be safely ignored or removed.

When a queue is opened on queue manager QM1 for the first time, subscription messages for information about the queue are created and routed through QM1's SYSTEM.CLUSTER.TRANSMIT.QUEUE to each repository queue manager. If QM1 did not have active channel connections to all repository queue managers some of these subscription messages would not be transmitted to the intended repository queue managers. Since QM1 is no longer a member and CLUSTER1 no longer exists, these messages will never be delivered.